Fluoroscopy is an imaging technique used by physicians to obtain real-time images of internal structures of a patient through use of a fluoroscope. A fluoroscope generally consists of an x-ray source (e.g., x-ray tube) and a fluorescent screen, between which the patient is placed. X-rays imparted on the fluorescent screen render an image of the patient's body. In conventional fluoroscopy, an x-ray beam is continuously projected from the x-ray source through the patient onto the screen for a predetermined length of time, typically ranging from 0.5-1.0 second.
Pulsed fluoroscopy is a type of fluoroscopy in which the x-ray beam is pulsed on and off during the imaging. Pulsed fluoroscopy has been shown to reduce the amount of radiation exposure to the patient. In conventional pulsed fluoroscopy, the x-ray beam is switched on and off to generate a predetermined number of x-ray pulses. Thus, for example, the x-ray beam may be switched off 50% of the time to yield a pulse rate of 12.5 pulses/second. FIGS. 1A-1B illustrate conventional fluoroscopy and conventional pulsed fluoroscopy, respectively. In one study, radiation exposure was reduced by 75% using pulsed fluoroscopy with 7.5 pulses/sec. Results of this study are published at Hernandez, R J, and Goodsitt, M M, “Reduction of radiation dose in pediatric patients using pulsed fluoroscopy,” American Journal of Roentgenology, © 1996, Vol. 167, pp. 1247-1253.
A particular type of pulsed fluoroscopy is grid controlled pulsed fluoroscopy (GCPF). GCPF has been successful in further reducing x-ray exposure. GCPF involves a grid positioned inside the x-ray tube, whereby the grid acts like a valve in sharpening pulse edges. When pulse edges are sharpened, so-called “soft” radiation 106 (FIG. 1) is blocked. FIG. 1C illustrates exemplary pulses as they might appear in a GCPF system.
Even with use of pulses and a grid, patients and hospital personnel may still be exposed to harmful ionizing radiation. Government regulations directed at the medical industry set forth limits on the amount of radiation exposure that can be administered through fluoroscopy. In the United States, the legal limits generally range from 5 R/min to 20 R/min, depending on whether the fluoroscopy unit includes Automatic Exposure Rate Control (AERC) or an optional high-level control. Of course, manufacturers and users (e.g., hospitals, doctors, technicians, etc.) of fluoroscopes want to meet the legally mandated limits. However, preferably the fluoroscope would administer radiation at an exposure rate well below the legally mandated maximum, thereby reducing exposure to patients and hospital personnel, while still generating an image of sufficient quality for medical purposes.
Unfortunately, even within legal guidelines, some systems cannot deliver images of satisfactory quality, particularly for certain types of patients. As an example, suppose that 10R/min is the maximum radiation dosage allowed for a particular system, and that for an average sized patient, 90 kV of input power is sufficient to generate an image of good quality. However, a higher voltage may be required to obtain an image of satisfactory quality for a larger patient. For example, using the same system, 130 kV may be required in order to deliver an image of satisfactory quality for a large patient. Typical systems are designed to prevent exposure at above the legal limit. As a result, the system will prevent 130 kV input, which will result in a poor quality image for the larger patient.
Thus, a system and method are needed that are able to generate x-ray images of sufficient quality for a broader range of patients, while still staying within specified radiation exposure rates.